Mussel of the

Page last updated
6 January 2017

Mussel of the Month

The January 2017 Mussel of the Month is Plectomerus dombeyanus. Plectomerus is a monotypic genus from the western Gulf Coastal Plain of North America, including the lower Mississippi Basin.

Plectomerus dombeyanus
USNM 84103. Lake St. Joseph, MS (figured type of Unio trapezoides Lea).

Plectomerus is a monotypic genus. A single species in its own genus separate from all other genera,* and that classification has been widely accepted since Ortmann & Walker (1922). That work was among the earliest formal attempts to reconcile the current nomenclature of freshwater mussels with the International Code of Zoological Nomenclature.

We are generally skeptical of monotypic genera. All species are distinct from other species, but genera should reflect information about relationships — groups of species. Sometimes monotypic genera are helpful, but sometimes they are not (e.g., Arkansia). What makes P. dombeyanus so distinctive that two classical anatomists like Ortmann and Walker thought it should be set apart from genera like Amblema and Megalonaias?

Ortmann (1912) had regarded the species now classified in Amblema and Megalonaias as well as P. dombeyanus to belong in the genus Crenodonta based on their shared soft-anatomical characters and similar shell morphology. At the time, though, Ortmann had noted that Frierson (a source of Ortmann’s specimens) had reported that some females of P. dombeyanus brooded their larvae in only the outer gills. The other species of Crenodonta brooded in all four gills. In technical terms, some P. dombeyanus are apparently ectobranchus while others are tetragenous, like the other species of Crenodonta.

Frierson (1914) — an early apologist of crazy Frenchmen — argued that Amblema Rafinesque, 1820 had priority over Crenodonta of Schlüter, 1838. And then Utterback (1915) sequestered the species with zig-zag umbo sculpture into a new genus, Megalonaias. This splitting of mussels with plicate (i.e., broadly wrinkled) shells into new genera forced Ortmann & Walker (1922) to choose a genus for P. dombeyanus. And, they decided — given Frierson’s claim of possibly different brooding anatomy — that it would be better to resurrect Plectomerus Conrad, 1853** than to make the classify it one of the other genera.

Fortunately, Ortmann & Walker (1922) actually fell backwards into the correct answer — or at least the “correctest” answer to-date. P. dombeyanus is not closely related to Megalonaias (Tribe Quadrulini). In fact, in some analyses, P. dombeyanus shares a more recent common ancestor with Lampsilis (Tribe Lampsilini) than Amblema (Tribe Amblemini) (Chapman et al., 2008). Plectomerus dombeyanus is a species with unclear phylogenetic affinities, and to park it in another genus would be more confusing.

Plectomerus is monotypic, and that is the way we like it.

* Frierson (1927) and others regarded Plectomerus as a subgenus of Amblema, but the sentiment is still the same.

** Conrad (1853) clearly intended Plectomerus as a genus equal in scope to the old Crenodonta, and it included the species now classified as Plectomerus, Amblema, and Megalonaias. However, Conrad neglected to assign a type species, and Ortmann & Walker (1922) applied their ICZN-given right to just pick one. The alternative would have been to pull from obscurity another questionably recognizable Rafinesque† genus, Bariosta Rafinesque, 1820.

† “Rafinesque” is used here as an adjective to describe poorly-described, should-have-been-suppressed taxa. We hope the practice catches on.


Phylum Mollusca
Class Bivalvia
Subclass Palaeoheterodonta
Order Unionoida

Family UNIONIDAE Rafinesque, 1820
Subfamily AMBLEMINAE Rafinesque, 1820
Tribe Amblemini Rafinesque, 1820

Genus Plectomerus Conrad, 1853

Species Plectomerus dombeyanus (Valenciennes, 1827)
(+ Unio trapezoides Lea, 1831)

To find out more about Plectomerus and its classification, check out:
  • Conrad, T.A. 1853. A synopsis of the family of Naïades of North America, with notes, and a table of some of the genera and sub-genera of the family, according to their geographical distribution, and descriptions of genera and sub-genera. Proceedings of the Academy of Natural Sciences of Philadelphia 6: 243-269.
  • Chapman, E.G., M.E. Gordon, J.M. Walker, B.K. Lang, D.C. Campbell & W.R. Hoeh. 2008. Evolutionary relationships of Popenaias popeii and the early evolution of lampsiline bivalves (Unionidae): phylogenetic analyses of DNA and amino acid sequences from F and M mitochondrial genomes. Malacologia 50(1-2): 303-318.
  • Frierson, L.S. 1914. Remarks on classification of the Unionidae. Nautilus 28: 6-8.
  • Frierson, L.S. 1927. A classification and annotated check list of the North American naiades. Baylor University Press, Waco, Texas. 111 pp.
  • Ortmann, A.E. 1912. Notes upon the families and genera of the najades. Annals of the Carnegie Museum 8: 222-365.
  • Ortmann, A.E. & B. Walker. 1922. On the nomenclature of certain North American naiades. Occasional Papers of the Museum of Zoology, University of Michigan (112). 75 pp.
  • Utterback, W.I. 1915. The naiades of Missouri. II. American Midland Naturalist 4(4): 97-152.
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